Field of the Invention
The present invention generally relates to a copper alloy sheet and a method for producing the same. More specifically, the invention relates to a sheet of a copper alloy containing nickel and silicon (a sheet of a Cu—Ni—Si alloy), which is used as the material of electric and electronic parts, such as connectors, lead frames, relays and switches, and a method for producing the same.
Description of the Prior Art
The materials used for electric and electronic parts as the materials of current-carrying parts, such as connectors, lead frames, relays and switches, are required to have a good electric conductivity in order to suppress the generation of Joule heat due to the carrying of current, as well as such a high strength that the materials can withstand the stress applied thereto during the assembly and operation of electric and electronic apparatuses using the parts. The materials used for electric and electronic parts, such as connectors, are also required to have an excellent bending workability since the parts are generally formed by bending after press blanking. Moreover, in order to ensure the contact reliability between electric and electronic parts, such as connectors, the materials used for the parts are required to have an excellent stress relaxation resistance, i.e., a resistance to such a phenomenon (stress relaxation) that the contact pressure between the parts is deteriorated with age.
Particularly in recent years, there is a tendency for electric and electronic parts, such as connectors, to be integrated, miniaturized and lightened. In accordance therewith, the sheets of copper and copper alloys serving as the materials of the parts are required to be thinned, so that the required strength level of the materials is more severe. Specifically, the tensile strength of the materials is desired to be the strength level of not less than 700 MPa, preferably not less than 750 MPa, and more preferably not less than 800 MPa.
However, there is generally a trade-off relationship between the strength and bending workability of a copper alloy sheet, so that it is difficult to obtain a copper alloy sheet satisfying both of the desired strength and bending workability as the required strength level of the material is more severe. In the case of a typical copper alloy sheet manufactured by rolling operations, it is known that the bending workability of the sheet in a bad way bending, in which the bending axis of the sheet is a rolling direction (LD), is greatly different from that in a good way bending in which the bending axis of the sheet is a direction (TD) perpendicular to the rolling direction and thickness direction. That is, it is known that the anisotropy of the bending workability of the copper alloy sheet is great. In particular, copper alloy sheets used as the materials of electric and electronic parts, such as connectors, which are small and have complicated shapes, are often formed by both of the good way bending and bad way bending. Therefore, it is strongly desired that the strength level of a copper alloy sheet is not only enhanced, but the anisotropy of the bending workability of the copper alloy sheet is also improved.
In addition, with the increase of cases where electric and electronic parts, such as connectors, are used in severe environments, the requirements for the stress relaxation resistance of copper alloy sheets used for the materials of the parts are more severe. For example, the stress relaxation resistance of electric and electronic parts, such as connectors, is particularly important when the parts are used for automobiles in high-temperature environments. Furthermore, the stress relaxation resistance is such a kind of creep phenomenon that the contact pressure on a spring portion of a material forming electric and electronic parts, such as connectors, is deteriorated with age in a relatively high-temperature (e.g., 100 to 200° C.) environment even if it is maintained to be a constant contact pressure at ordinary temperature. That is, the stress relaxation resistance is such a phenomenon that the stress applied to a metal material is relaxed by plastic deformation produced by the movement of dislocation, which is caused by the self-diffusion of atoms forming a matrix and the diffusion of the solid solution of atoms, in such a state that the stress is applied to the metal material.
However, there are generally trade-off relationships between the strength and electric conductivity of a copper alloy sheet and between the bending workability and stress relaxation resistance thereof, in addition to the above-described trade-off relationship between the strength and bending workability thereof. Therefore, conventionally, a copper alloy sheet having a good strength, bending workability or stress relaxation resistance is suitably chosen in accordance with the use thereof as a material used for a current-carrying part, such as a connector.
Among copper alloy sheets used for the materials of electric and electronic parts, such as connectors, the sheets of Cu—Ni—Si alloys (so-called Corson alloys) are noted as materials having a relatively excellent characteristic balance between the strength and electric conductivity thereof. For example, the sheets of Cu—Ni—Si alloys can have the strength of not less than 700 MPa while maintaining a relatively high electric conductivity (30 to 50% IACS) by a process basically comprising a solution treatment, cold-rolling, ageing treatment, finish cold-rolling and low-temperature annealing. However, the bending workability of the sheets of Cu—Ni—Si alloys is not always good since they have a high strength.
As methods for improving the strength of the sheets of Cu—Ni—Si alloys, there are known a method for increasing the amount of solute elements, such as Ni and Si, to be added, and a method for enhancing a rolling reduction in a finish rolling (temper rolling) operation after an ageing treatment. However, in the method for increasing the amount of solute elements, such as Ni and Si, to be added, the electric conductivity of the sheets of the alloys is deteriorated, and the amount of Ni—Si deposits is increased to easily deteriorate the bending workability thereof. On the other hand, in the method for enhancing the rolling deduction in the finish rolling operation after the ageing treatment, the extent of work hardening is enhanced to remarkably deteriorate the bad way bending workability, so that there are some cases where the sheets can not be worked as electric and electronic parts, such as connectors, even if the strength and electric conductivity thereof are high.
As a method for preventing the deterioration of the bending workability of the sheets of Cu—Ni—Si alloys, there is known a method for omitting the finish cold-rolling after the ageing treatment or minimizing the cold-rolling reduction as well as compensating the deterioration of the strength of the sheets by increasing the amount of solute elements, such as Ni and Si, to be added thereto. However, in this method, there is a problem in that the bending workability in the good way is remarkably deteriorated.
In order to improve the bending workability of the sheets of copper alloys, a method for fining the crystal grains of the copper alloys is effective. This is the same in the case of the sheets of Cu—Ni—Si alloys. Therefore, the solution treatment for the sheets of Cu—Ni—Si alloys is often carried out in a relatively low temperature range so as to cause part of deposits (or crystallized substances) for pinning the growth of recrystallized grains to remain, not in a high temperature range in which all of the deposits (or crystallized substances) are caused to form the solid solution thereof. However, if the solution treatment is carried out in such a low temperature range, the strength level of the sheets after the ageing treatment is necessarily lowered since the amount of the solid solution of Ni and Si is decreased although the crystal grains can be fined. In addition, since the area of grain boundaries existing per a unit volume is increased as the crystal grain size is decreased, the fining of the crystal grains causes to promote stress relaxation being a kind of creep phenomenon. In particular, in sheets used as the materials of automotive connectors or the like in high-temperature environments, the diffusion rate along the grain boundaries of atoms is far higher than that in the grains, so that the deterioration of the stress relaxation resistance of the sheets due to grain refining causes a serious problem.
In recent years, as methods for improving such a problem on the bending workability of the sheets of Cu—Ni—Si alloys, there are proposed various methods for improving the bending workability of the sheets by controlling the crystal orientation (texture). For example, there are proposed a method for improving the bending workability of a sheet in the good way by causing (I{111}+I{311})/I{220}≤2.0 to be satisfied assuming that the intensity of the X-ray diffraction on a {hkl} plane is I{hkl} (see, e.g., Japanese Patent Laid-Open No. 2006-9108), and a method for improving the bending workability of a sheet in the bad way by causing (I{111}+I{311})/I{220}>2.0 to be satisfied assuming that the intensity of the X-ray diffraction on a {hkl} plane is I{hkl} (see, e.g., Japanese Patent Laid-Open No. 2006-16629). There is also proposed a method for improving the bending workability of the sheets of Cu—Ni—Si alloys by causing the sheets to have a mean crystal grain size of 10 μm or less and such a texture that the percentage of the Cube orientation {001}<100>, which is known as one of recrystallized textures, is 50% or more in the results of measurement based on the SEM-EBSP method (see, e.g., Japanese Patent Laid-Open No. 2006-152392). In addition, there is proposed a method for improving the bending workability of the sheets of Cu—Ni—Si alloys by causing (I{200}+I{311})/I{220}≥0.5 to be satisfied (see, Japanese Patent Laid-Open No. 2000-80428). Moreover, there is proposed a method for improving the bending workability of the sheet of a Cu—Ni—Si alloy by causing I{311}×A/(I{311}+I{220}+I{200})<1.5 to be satisfied assuming that the crystal grain size of the sheet is A (μm) and that the intensities of X-ray diffraction from the {311}, {220} and {200} planes on the surface of the sheet are I{311}, I{220} and I{200}, respectively (see, Japanese Patent Laid-Open No. 2006-9137).
Furthermore, the pattern of X-ray diffraction from the surface (rolled surface) of the sheet of a Cu—Ni—Si alloy generally comprises the peaks of diffraction on five crystal planes of {111}, {200}, {220}, {311} and {422}. The intensities of X-ray diffraction from other crystal planes are far smaller than those from the five crystal planes. The intensities of X-ray diffraction on the {200}, {311} and {422} planes are usually increased after a solution treatment (recrystallization). The intensities of X-ray diffraction on these planes are decreased by the subsequent cold rolling operation, so that the intensity of X-ray diffraction on the {220} plane is relatively increased. Usually, the intensity of X-ray diffraction on the {111} plane is not so varied by the cold rolling operation. Therefore, in the above described Japanese Patent Laid-Open Nos. 2006-9108, 2006-16629, 2006-152392, 2000-80428 and 2006-9137, the crystal orientation (fixture) of Cu—Ni—Si alloys is controlled by the intensities of X-ray diffraction from these crystal planes.
However, in the method disclosed in Japanese Patent Laid-Open No. 2006-9108, the bending workability of a sheet in the good way is improved by causing (I{111}+I{311})/I{220}≤2.0 to be satisfied, whereas in the method disclosed in Japanese Patent Laid-Open No. 2006-16629, the bending workability of a sheet in the bad way by causing (I{111}+I{311})/I{220}>2.0 to be satisfied, so that the conditions of the improvement of the bending workability of a sheet in the good way is reverse to those in the bad way. Therefore, it is difficult to improve the bending workability of a sheet in both of the good and bad ways by the methods disclosed in Japanese Patent Laid-Open Nos. 2006-9108 and 2006-16629.
In the method disclosed in Japanese Patent Laid-Open No. 2006-152392, the stress relaxation resistance of the sheets is often deteriorated since it is required to fine the crystal grains of the sheets to cause the sheets to have a mean crystal grain size of 10 μm or less.
In the method disclosed in Japanese Patent Laid-Open No. 2000-80428, it is required to decrease the percentage of the {220} crystal plane, which is the principal orientation of rolling texture, so as to cause (I{200}+I{311})/I{220}≥0.5 to be satisfied. For that reason, if the rolling reduction in the cold rolling after the solution treatment is decreased, it is possible to improve the bending workability of the sheets. However, if the sheets are so controlled as to have such a rolling texture, the strength of the sheets is often decreased, so that the tensile strength thereof is about 560 to 670 MPa.
In the method disclosed in Japanese Patent Laid-Open No. 2006-9137, it is required to fine the crystal grains in order to improve the bending workability of the sheet, so that the stress relaxation resistance of the sheet is often deteriorated.
As described above, although a method for fining the crystal grains of a copper alloy sheet is effective in order to improve the bending workability of the sheet, the stress relaxation resistance of the sheet is deteriorated by fining the crystal grains of the sheet, so that it is difficult to improve both of the bending workability and stress relaxation resistance of the sheet.